The invention relates to a method and a device for welding plastic pipes made of thermoplastic material by means of a butt-welding process, the pipes being held in a coaxial position in relation to one another by means of clamping fixtures, trimmed by means of a trimmer which can be brought between the clamping fixtures and heated up by means of heated tools which can be fitted in place of the trimmer, the heating of the free pipe ends to be welded having the effect of producing a melting region, which forms a bead when pressing occurs.
The known method for the butt-welding of two plastic pipes has so far always proceeded in substantially the same way, whether performed by manual or automatic machines. The plastic pipes are clamped in the clamping fixtures provided for them, the opposite pipe ends projecting from the clamping fixtures and all damage and soiling being removed by cutting with the trimmer located between the two pipe ends and moving the clamping fixtures together, in the direction of the rotating trimmer. In addition, the effect is achieved that the end faces of the pipe ends are at right angles to the axis of the pipe and parallel to the opposite end face of the other pipe end. Subsequently, the pipe ends are moved apart and the trimmer removed, after which a heated tool is brought between the pipes, whereupon the clamping fixtures are moved together again in order to bring the end faces of the pipe ends into contact with the heated tool. The pipe ends are brought into contact with the heated tool under a predetermined, constant pressure, which is dependent on the material, the pipe diameter and the wall thickness. The pressure, which is usually generated by means of a hydraulic unit, is exerted on the two pipe ends or the heated tool until beads corresponding to the predetermined dimensions are produced on the circumference of the pipe ends. In the subsequent heating-up phase, the pressure is reduced and maintained over a prescribed time in order to achieve the temperature profile that is required for welding in the joining zone of the plastic pipe. After that, the clamping fixtures are pushed apart again, the heated tool is removed and the pipe ends are subsequently pressed together under a predetermined pressure and for a defined time period, so that a required penetration of the pipe ends takes place and welding of the pipe ends is ensured. With this type of production of welded connections of plastic pipes, the beads are not optimized in their size and tend to be large, which has an adverse influence on the flow in the pipe and also requires long process times.
DE 198 27 146 A1 discloses a method for the butt-welding of plastic pipes, the clamping fixtures being driven by an electric motor which is assigned a controller. Starting from the position in which the two pipe ends are in contact with the heated tool, the clamping fixtures move in accordance with a predetermined time-displacement curve which is stored in the controller, is assigned a required displacement for bringing the ends into contact with the heated tool and is substantially stopped during the heating-up phase. After the removal of the heated tools, the clamping fixtures undergo a specific displacement, required for joining, in accordance with a second predetermined time-displacement curve, starting from the position in which the two pipe ends come into contact with one another at the beginning of the joining process, and are substantially stopped during the cooling-down phase.
A disadvantage of such a solution is that the method requires a complex controller, in which there is stored for each diameter and each material a corresponding time-displacement curve. Moreover, the device requires feelers or sensors for determining the position of the pipe ends, which must be determined twice during a welding operation.
The object of the invention is to propose a method and an associated device that make it possible for pipe ends to be welded with minimal bead formation, while maintaining safety conditions, and minimize energy and time consumption and moreover manage with a low-cost and simple controller.